Hydraulic motors of the state of the art generally comprise a servo control system with a servo piston slidable disposed in a servo cylinder. The servo piston acts via piston rod on an adjustment element for adjusting the stroke of the hydraulic motor. The position of the servo piston is controlled by supplying or draining hydraulic fluid to or from an end face of the servo piston under control of a control unit acting on a control valve. Such a hydraulic unit featuring a two-position variable axial piston hydraulic motor is described in Chinese Patent CN 101871477 A, for example.
Two-position hydraulic motors are usually designed for low cost and small package size. Thereby a shifting pressure derived usually from a system pressure higher than a case pressure acts at least on the first front face of the servo piston. However, if the shifting pressure level fluctuates around a predetermined pressure level for shifting the servo piston to either position of the two end positions, the servo piston changes abruptly from one end position in the servo cylinder to the other, i.e. the servo piston changes its position without intermediate positions from its initial position abruptly into the operated position and vice versa. Therefore abrupt changes in the displacement of the motor are caused, which are difficult to control. On one hand quick and controlled changes in the servo piston position are desired to achieve short reaction times of the hydraulic unit. On the other hand these abrupt changes are caused too by unsteady system pressures acting immediately on one of the front faces of the servo piston. In common designs changes in the high pressure or system pressure influence the shifting pressure directly, wherein these changes in the high pressure or in the system pressure can be/are triggered also by work load changes in operation of the hydraulic unit. This makes a control of the shifting performance and the propel/breaking behaviour of known hydraulic units even more difficult.
These abrupt changes in the displacement volume of the driving unit leads to an unsteady, non-smooth behaviour of the hydraulic unit and the propel unit associated therewith, especially with regard to the breaking/acceleration behaviour of two-position hydraulic units. In two-position units the initial servo piston position defines either the maximum displacement volume of the drive unit of the hydraulic unit, and the operated servo piston position defines the minimum displacement volume or vice versa. This means further that the changes of position of the servo piston causes the hydraulic unit to change from maximum performance to minimum and vice versa. Such that abrupt changes in the performance are received by the operator of a hydraulic unit like a “digital” stop and go behaviour which makes him difficult to control the hydraulic application.
Further, abrupt changes in the performance of a hydraulic unit require a robust design of the support/mounting elements within the hydraulic unit, which have to bear the high and quickly changing alternating forces. This in turn leads to respective big components and package sizes as casings, bearings, bearing zones, etc., have to be designed accordingly robust and/or massive. This often leads furthermore, especially when using low-cost materials, to respective heavy-weight hydraulic units.